Like Yukino, Arima has also been living his life underneath a mask, but for different reasons. He had a troubled childhood, and tried to forget about the past by becoming someone respectable.
But meeting Yukino brought back all those painful memories. Underneath his cool exterior, he is constantly fighting the dark side of himself, threatening his happiness.
Arima had a traumatic childhood, and the only thing he remembers about his real parents is the memory of them hitting him. He was always running away and crying. His parents physically and emotionally abused little Arima and eventually disappeared, leaving him in the care of his uncle and his wife.
The Arima clan came together for a meeting, and bluntly stated that Arima would probably become just like his parents. But his new parents took him in with gentle hands, and loved him unconditionally. So Arima felt the need to pay them back by becoming perfect to make them proud of him.
Arima controls his emotions very well, and rarely lets his real emotions show. His stepparents are very loving, and would want nothing more than for Arima to open up to them. But he could never openly talk to them like the way Yukino does with her crazy family. He is super polite with them, and does not want to be a hassle to them.
He comes from a wealthy line of doctors, and his stepfather is the director of the hospital. With such an impresive background, he could've easily gone to any top private school. But he doesn't feel comfortable asking for anything more than what his stepparents had already given him.
Arima is the most popular guy at school, along with Asaba. With his easygoing personality and good looks, all the girls are in love with him, and all the guys want to be just like him. He also has the respect of the teachers, as their most promising student.
He has a lot of charisma, and wherever he goes, people are attracted to him. He excels in the sport of kendo, and is the star on the school's kendo club. He is so popular that when his team goes to the other schools to compete, everybody - including the other team's fans - cheer him on.
He is involved in even more committees and extracurricular activities than Yukino, so he is always super busy. He plans to follow in his father's footsteps into the medical field, and hopes to attend the prestigious Tokyo University.